Add ADVSIMD64 optimizations for System.Text.Encodings.Web

[GrabYourPitchforks]

This is a follow-up PR to #49373 which adds ARM64 SIMD optimizations to the FindFirstCharacterToEncode code paths. I'm not able to run BenchmarkDotNet on my SPX device for some reason, but I created a small console application to get some quick measurements.

Baseline (release/5.0): 46 ns for HtmlEncoder.Default.FindFirstCharacterToEncodeUtf8(u8"The quick brown fox jumps over the lazy dog.")
advsimd-optimized: 8.8 ns for same input. (-81% wall clock time taken)

The .AdvSimd64.cs file is basically a carbon copy of the .Ssse3.cs file, but using AMD64 patterns (tbl and addp) instead of SSSE3 instructions (pshufb and pmovmskb).

We're relying on the increased unit test coverage given by the previous PR. I've also augmented the unit tests with an extra "insert this scurrilous BMP code point" check to validate that our UTF-16 -> ASCII conversion is working properly.

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Issue Details

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This is a follow-up PR to #49373 which adds ARM64 SIMD optimizations to the FindFirstCharacterToEncode code paths. I'm not able to run BenchmarkDotNet on my SPX device for some reason, but I created a small console application to get some quick measurements.

Baseline (release/5.0): 46 ns for HtmlEncoder.Default.FindFirstCharacterToEncodeUtf8(u8"The quick brown fox jumps over the lazy dog.")
advsimd-optimized: 8.8 ns for same input. (-81% wall clock time taken)

The .AdvSimd64.cs file is basically a carbon copy of the .Ssse3.cs file, but using AMD64 patterns (tbl and addp) instead of SSSE3 instructions (pshufb and pmovmskb).

We're relying on the increased unit test coverage given by the previous PR. I've also augmented the unit tests with an extra "insert this scurrilous BMP code point" check to validate that our UTF-16 -> ASCII conversion is working properly.

Author:	GrabYourPitchforks
Assignees:	-
Labels:	area-System.Text.Encodings.Web
Milestone:	-

[GrabYourPitchforks]

/cc @adamsitnik @DrewScoggins

Heads up that you're likely to see changes in the perf lab benchmark here and in System.Text.Json as a result of recent PRs.

• System.Text.Encodings.Web refactoring and code modernization #49373 (previous PR, now merged) likely improved x64 performance but regressed arm64 performance
• This current PR should make up for the arm64 loss caused by the previous PR

[eiriktsarpalis]

Thanks

[DrewScoggins]

Thanks for the tag @GrabYourPitchforks. We will keep an eye out for this on Tuesday when we triage.

[GrabYourPitchforks]

Green CI? Sure, I'll take it!
(If anybody has any extra comments, open a new issue or ping me offline and we can get them addressed in a future PR.)

[adamsitnik]

I'm not able to run BenchmarkDotNet on my SPX device for some reason

@GrabYourPitchforks what error are you getting?